THE NEOZOIC AGES.
Between the Mesozoic and the next succeeding time which may be known
as the Neozoic or Tertiary,[AG] there is in the arrangements of most
geologists a great break in the succession of life; and undoubtedly
the widespread and deep subsidence of the Cretaceous, followed by the
elevation of land on a great scale at the beginning of the next
period, is a physical cause sufficient to account for vast life
changes. Yet we must not forget to consider that even in the
Cretaceous itself there were new features beginning to appear. Let us
note in this way, in the first place, the introduction of the familiar
generic forms of exogenous trees. Next we may mention the decided
prevalence of the modern types of coral animals and of a great number
of modern generic forms of mollusks. Then we have the establishment of
the modern tribes of lobsters and crabs, and the appearance of nearly
all the orders of insects. Among vertebrates, the ordinary fishes are
now introduced. Modern orders of reptiles, as the crocodiles and
chelonians, had already appeared, and the first mammals. Henceforth
the progress of organic nature lies chiefly in the dropping of many
Mesozoic forms and in the introduction of the higher tribes of mammals
and of man.
[AG] The former name is related to Palæozoic and Mesozoic, the latter
to the older terms Primary and Secondary. For the sake of euphony we
shall use both. The term Neozoic was proposed by Edward Forbes for the
Mesozoic and Cainozoic combined; but I use it here as a more
euphonious and accurate term for the Cainozoic alone.
It is further to be observed that the new things introduced in the
later Mesozoic came in little by little in the progress of the period,
and anticipated the great physical changes occurring at its close. On
the other hand, while many family and even generic types pass over
from the Mesozoic to the earlier Tertiary, very few species do so. It
would seem, therefore, as if changes of species were more strictly
subordinate to physical revolutions then were changes of genera and
orders--these last overriding under different specific forms many
minor vicissitudes, and only in part being overwhelmed in the grander
revolutions of the earth.
Both in Europe and America there is evidence of great changes of level
at the beginning of the Tertiary. In the west of Europe beds often of
shallow-water or even fresh-water origin fill the hollows in the bent
Cretaceous strata. This is manifestly the case with the formations of
the London and Paris basins, contemporaneous but detached deposits of
the Tertiary age, lying in depressions of the chalk. Still this does
not imply much want of conformity, and according to the best explorers
of those Alpine regions in which both the Mesozoic and Tertiary beds
have been thrown up to great elevations, they are in the main
conformable to one another. Something of the same kind occurs in
America. On the Atlantic coast the marine beds of the Older Tertiary
cover the Cretaceous, and little elevation seems to have occurred
Farther west the elevation increases, and in the upper part of the
valley of the Mississippi it amounts to 1700 feet. Still farther west,
in the region of the Rocky Mountains, there is evidence of elevation
to the extent of as much as 7000 feet. Throughout all these regions
scarcely any disturbance of the old Cretaceous sea-bottom seems to
have occurred until after the deposition of the older Tertiary, so
that there was first a slow and general elevation of the Cretaceous
ocean bottom, succeeded by gigantic folds and fractures, and extensive
extravasations of the bowels of the earth in molten rocks, in the
course of the succeeding Tertiary age. These great physical changes
inaugurated the new and higher life of the Tertiary, just as the
similar changes in the Permian did that of the Mesozoic.
The beginning of these movements consisted of a great and gradual
elevation of the northern parts of both the Old and New Continents out
of the sea, whereby a much greater land surface was produced, and such
changes of depth and direction of currents in the ocean as must have
very much modified the conditions of marine life. The effect of all
these changes in the aggregate was to cause a more varied and variable
climate, and to convert vast areas previously tenanted by marine
animals into the abodes of animals and plants of the land, and of
estuaries, lakes, and shallow waters. Still, however, very large
areas now continental were under the sea. As the Tertiary period
advanced, these latter areas were elevated, and in many cases were
folded up into high mountains. This produced further changes of
climate and habitat of animals, and finally brought our continents
into all the variety of surface which they now present, and which fits
them so well for the habitation of the higher animals and of man.
The thoughtful reader will observe that it follows from the above
statements that the partial distribution and diversity in different
localities which apply to the deposits of such ages as the Permian and
the Trias apply also to the earlier Tertiary; and as the continents,
notwithstanding some dips under water, have retained their present
forms since the beginning of the Tertiary, it follows that these beds
are more definitely related to existing geographical conditions then
are those of the older periods, and that the more extensive marine
deposits of the Tertiary are, to a great extent, unknown to us. This
has naturally led to some difficulty in the classification of Neozoic
deposits--those of some of the Tertiary ages being very patchy and
irregular, while others spread very widely. In consequence of this,
Sir Charles Lyell, to whom we owe very much of our definite knowledge
of this period, has proposed a subdivision based on the percentage of
recent and fossil animals. In other words, he takes it for granted
that a deposit which contains more numerous species of animals still
living then another, may be judged on that account to be more recent.
Such a mode of estimation is, no doubt, to some extent arbitrary; but
in the main, when it can be tested by the superposition of deposits,
it has proved itself reliable. Further, it brings before us this
remarkable fact, that while in the older periods all the animals whose
remains we find are extinct as species, so soon as we enter on the
Neozoic we find some which still continue to our time--at first only a
very few, but in later and later beds in gradually increasing
percentage, till the fossil and extinct wholly disappear in the recent
and living.
The Lyellian classification of the Tertiary will therefore stand as in
the following table, bearing in mind that the percentage of fossils is
taken from marine forms, and mainly from mollusks, and that the system
has in some cases been modified by stratigraphical evidence:--
{ Post-pliocene, including that which immediately
{ precedes the Modern. In this the shells, etc.,
{ are recent, the Mammalia in part extinct.
{
{ Pliocene, or more recent age. In this the
{ majority of shells found are recent in the
Tertiary, or { upper beds. In the lower beds the extinct
Neozoic Time. { become predominant.
{
{ Miocene, or less recent. In this the large
{ majority of shells found are extinct.
{
{ Eocene, the dawn of the recent. In this only
{ a few recent shells occur.
If we attempt to divide the Tertiary time into ages corresponding to
those of the older times, we are met by the difficulty that as the
continents have retained their present forms and characters to a great
extent throughout this time, we fail to find those evidences of
long-continued submergences of the whole continental plateaus, or very
large portions of them, which we have found so very valuable in the
Palæozoic and Mesozoic. In the Eocene, however, we shall discover one
very instructive case in the great Nummulitic Limestone. In the
Miocene and Pliocene the oscillations seem to have been slight and
partial. In the Post-pliocene we have the great subsidence of the
glacial drift; but that seems to have been a comparatively rapid dip,
though of long duration when measured by human history; not allowing
time for the formation of great limestones, but only of fossiliferous
sands and clays, which require comparatively short time for their
deposition If then we ask as to the duration of the Neozoic, I answer
that we have not a definite measure of its ages, if it had any; and
that it is possible that the Neozoic may have as yet had but one age,
which closed with the great drift period, and that we are now only in
the beginning of its second age. Some geologists, impressed with this
comparative shortness of the Tertiary, connect it with Mesozoic,
grouping both together. This, however, is obviously unnatural. The
Mesozoic time certainly terminated with the Cretaceous, and what
follows belongs to a distinct aeon.
But we must now try to paint the character of this new and peculiar
time; and this may perhaps be best done in the following sketches: 1.
The seas of the Eocene. 2. Mammals from the Eocene to the Modern. 3.
Tertiary floras. 4. The Glacial period. 5. The Advent of Man.
The great elevation of the continents which closed the Cretaceous was
followed by a partial and unequal subsidence, affecting principally
the more southern parts of the land of the northern hemisphere. Thus,
a wide sea area stretched across all the south of Europe and Asia, and
separated the northern part of North America from what of land existed
in the southern hemisphere. This is the age of the great Nummulitic
Limestones of Europe, Africa, and Asia, and the Orbitoidal Limestones
of North America. The names are derived from the prevalence of certain
forms of those humble shell-bearing protozoa which we first met with
in the Laurentian, and which we have found to be instrumental in
building up the chalk, the _Foraminifera_ of zoologists. (Fig. p.
243.) But in the Eocene the species of the chalk were replaced by
certain broad flat forms, the appearance of which is expressed by the
term nummulite, or money-stone; the rock appearing to be made up of
fossils, somewhat resembling shillings, sixpences, or three-penny
pieces, according to the size of the shells, each of which includes a
vast number of small concentric chambers, which during life were
filled with the soft jelly of the animal. The nummulite limestone was
undoubtedly oceanic, and the other shells contained in it are marine
species. After what we have already seen we do not need this
limestone to convince us of the continent-building powers of the
oceanic protozoa; but the distribution of these limestones, and the
elevation which they attain, furnish the most striking proofs that we
can imagine of the changes which the earth's crust has undergone in
times geologically modern, and also of the extreme newness of man and
his works. Large portions of those countries which constitute the
earliest seats of man in Southern Europe, Northern Africa, and Western
and Southern Asia, are built upon the old nummulitic sea-bottom. The
Egyptians and many other ancient nations quarried it for their oldest
buildings. In some of these regions it attains a thickness of several
thousand feet, evidencing a lapse of time in its accumulation equal to
that implied in the chalk itself. In the Swiss Alps it reaches a
height above the sea of 10,000 feet, and it enters largely into the
structure of the Carpathians and Pyrenees. In Thibet it has been
observed at an elevation of 16,500 feet above the sea. Thus we learn
that at a time no more geologically remote then the Eocene Tertiary,
lands now of this great elevation were in the bottom of the deep sea;
and this not merely for a little time, but during a time sufficient
for the slow accumulation of hundreds of feet of rock, made up of the
shells of successive generations of animals. If geology presented to
us no other revelation then this one fact, it would alone constitute
one of the most stupendous pictures in physical geography which could
be presented to the imagination. I beg leave here to present to the
reader a little illustration of the limestone-making Foraminifera of
the Cretaceous and Eocene seas. In the middle above is a nummulite of
the natural size. Below is another, sliced to show its internal
chambers. At one side is a magnified section of the common building
stone of Paris, the milioline limestone of the Eocene, so called from
its immense abundance of microscopic shells of the genus Miliolina. At
the other side is a magnified section of one of the harder varieties
of chalk, ground so thin as to become transparent,[AH] and mounted in
Canada balsam. It shows many microscopic chambered shells of
Foraminifera. These may serve as illustrations of the functions of
these humble inhabitants of the sea as accumulators of calcareous
matter. It is further interesting to remark that some of the beds of
nummulitic limestone are so completely filled with these shells, that
we might from detached specimens suppose that they belonged to
sea-bottoms whereon no other form of life was present. Yet some beds
of this age are remarkably rich in other fossils. Lyell states that as
many as six hundred species of shells have been found in the principal
limestone of the Paris basin alone; and the lower Eocene beds afford
remains of fishes, of reptiles, of birds, and of mammals. Among the
latter are the bones of gigantic whales, of which one of the most
remarkable is the Zeuglodon of Alabama, a creature sometimes seventy
feet in length, and which replaces in the Tertiary the great
Elasmosaurs and Ichthyosaurs of the Mesozoic, marking the advent, even
in the sea, of the age of Mammals as distinguished from the age of
Reptiles.
[AH] As for instance that of the Giant's Causeway, Antrim.
[Illustration: FORAMINIFERAL ROCK-BUILDERS.
A. Nummulites lævigata--Eocene.
B. The same, showing chambered interior.
C. Milioline limestone, magnified--Eocene, Paris.
D. Hard Chalk, section magnified--Cretaceous.]
This fact leads us naturally to consider in the second place the
mammalia, and other land animals of the Tertiary. At the beginning of
the period we meet with that higher group of mammals, not pouched,
which now prevails. Among the oldest of these Tertiary beasts are
_Coryphodon_, an animal related to the Modern Tapirs, and _Arctocyon_,
a creature related to the bears and racoons. These animals represent
respectively the Pachyderms, or thick-skinned mammals, and the
ordinary Carnivora. Contemporary with or shortly succeeding these,
were species representing the Rodents, or gnawing animals, and many
other creatures of the group Pachydermata, allied to the Modern Tapirs
and Hogs, as well as several additional carnivorous quadrupeds. Thus
at the very beginning of the Tertiary period we enter on the age of
mammals, It may be well, however, to take these animals somewhat in
chronological order.
If the old Egyptian, by quarrying the nummulite limestone, bore
unconscious testimony to the recent origin of man (whose remains are
wholly absent from the Tertiary deposits), so did the ancient Britons
and Gauls, when they laid the first rude foundations of future
capitals on the banks of the themes and of the Seine. Both cities lie
in basins of Eocene Tertiary, occupying hollows in the chalk. Under
London there is principally a thick bed of clay, the "London clay"
attaining a thickness of five hundred feet. This bed is obviously
marine, containing numerous species of sea shells; but it must have
been deposited near land, as it also holds many fossil fruits and
other remains of plants to which we shall refer in the sequel, and the
bones of several species of large animals. Among these the old
reptiles of the Mesozoic are represented by the vertebrae of a
supposed "sea snake" (Palæophis) thirteen feet long, and species of
crocodile allied both to the alligators and the gavials. But besides
these there are bones of several animals allied to the hog and tapir,
and also a species of opossum, These remains must be drift carcases
from neighbouring shores, and they show first the elevation of the
old deep-sea bottom represented by the chalk, so that part of it
became dry land; next, the peopling of that land by tribes of animals
and plants unknown to the Mesozoic; and lastly, that a warm climate
must have existed, enabling England at this time to support many types
of animals and plants now proper to intertropical regions. As Lyell
well remarks, it is most interesting to observe that these beds belong
to the beginning of the Tertiary, that they are older then those great
nummulite limestones to which we have referred, and that they are
older then the principal mountain chains of Europe and Asia. They show
that no sooner was the Cretaceous sea dried from off the new land,
then there were abundance of animals and plants ready to occupy it,
and these not the survivors of the flora and fauna of the Wealden, but
a new creation. The mention of the deposit last named places this in a
striking light. We have seen that the Wealden beds, under the chalk,
represent a Mesozoic estuary, and in it we have the remains of the
animals and plants of the land that then was. The great Cretaceous
subsidence intervened, and in the London clay we have an estuary of
the Eocene. But if we pass through the galleries of a museum where
these formations are represented, though we know that both existed in
the same locality under a warm climate, we see that they belong to two
different worlds, the one to that of the Dinosaurs, the Ammonites, the
Cycads, and the minute Marsupials of the Mesozoic, the other to that
of the Pachyderms, the Palms, and the Nautili of the Tertiary.
The London clay is lower Eocene; but in the beds of the Isle of Wight
and neighbouring parts of the South of England, we have the middle and
upper members of the series. They are not, however, so largely
developed as in the Paris basin, where, resting on the equivalent of
the London clay, we have a thick marine limestone, the Calcaire
Grossier, abounding in marine remains, and in some beds composed of
shells of foraminifera. The sea in which this limestone was deposited,
a portion no doubt of the great Atlantic area of the period, became
shallow, so that beds of sand succeeded those of limestone, and
finally it was dried up into lake basins, in which gypsum, magnesian
sediments, and siliceous limestone were deposited. These lakes or
ponds must at some period have resembled the American "salt-licks,"
and were no doubt resorted to by animals from all the surrounding
country in search of the saline mud and water which they afforded.
Hence in some marly beds intervening between the layers of gypsum,
numerous footprints occur, exactly like those already noticed in the
Trias. Had there been a Nimrod in those days to watch with bow or
boomerang by the muddy shore, he would have seen herds of heavy
short-legged and three-hoofed monsters (Palæotherium), with large
heads and long snouts, probably scantily covered with sleek hair, and
closely resembling the Modern Tapirs of South America and India,
laboriously wading through the mud, and grunting with indolent delight
as they rolled themselves in the cool saline slime. Others more light
and graceful, combining some features of the antelope with those of
the Tapir (Anoplotherium) ran in herds over the drier ridges, or
sometimes timidly approached the treacherous clay, tempted by the
saline waters. Other creatures representing the Modern Damans or
Conies--"feeble folk" which, with the aspect of hares, have the
structure of Pachyderms--were also present. Creatures of these types
constituted the great majority of the animals of the Parisian Eocene
lakes; but there were also Carnivorous animals allied to the hyæna,
the wolf, and the opossum, which prowled along the shores by night to
seize unwary wanderers, or to prey on the carcases of animals mired in
the sloughs. Wading birds equal in size to the ostrich also stalked
through the shallows, and tortoises crawled over the mud.
Lyell mentions the discovery of some bones of one of these gigantic
birds (Gastornis) in a bed of the rolled chalk flints which form the
base of the Paris series, resting immediately on the chalk; one of the
first inhabitants perhaps to people some island of chalk just emerged
from the waters, and under which lay the bones of the mighty
Dinosaurs, and in which were embedded those of sea birds that had
ranged, like the albatross and petrel, over the wide expanse of the
Cretaceous ocean. These waders, however, like the tortoises and
crocodiles and small marsupial mammals, form a link of connection in
type at least between the Eocene and the Cretaceous, for bones of
wading birds have been found in the Greensands indicating their
existence before the close of the Mesozoic.
The researches of Baron Cuvier in the bones collected in the quarries
of Montmartre were regarded as an astonishing triumph of comparative
anatomy; and familiar as we now are with similar and yet more
difficult achievements, we can yet afford to regard with admiration
the work of the great French naturalist as it is recorded in its
collected form in his "Recherches sur les Ossemens Fossiles,"
published in 1812. His clear and philosophical views as to the plan
perceptible in nature, his admirable powers of classification, his
acute perception of the correlation of parts in animals, his nice
discrimination of the resemblances and differences of fossil and
recent structures, and of the uses of these,--all mark him as one of
the greatest minds ever devoted to the study of natural science. It is
obvious, that had his intellect been occupied by the evolutionist
metaphysics which pass for natural science with too many in our day,
he would have effected comparatively little; and instead of the
magnificent museum in the "Règne Animal" and the "Ossemens Fossiles,"
we might have had wearisome speculations on the derivation of species.
It is reason for profound thankfulness that it was not so; and also
that so many great observers and thinkers of our day, like Sedgwick,
Murchison, Lyell, Owen, Dana, and Agassiz, have been allowed to work
out their researches almost to completion before the advent of those
poisoned streams and mephitic vapours which threaten the intellectual
obscuration of those who should be their successors.
If we pass from the Eocene to the Miocene, still confining ourselves
mainly to mammalian life, we find three remarkable points of
difference--(1) Whereas the Eocene mammals are remarkable for
adherence to one general type, viz., that group of pachyderms most
regular and complete in its dentition, we now find a great number of
more specialised and peculiar forms; (2) We find in the latter period
a far greater proportion of large carnivorous animals; (3) We find
much greater variety of mammals then either in the Eocene or the
Modern, and a remarkable abundance of species of gigantic size. The
Miocene is thus apparently the culminating age of the mammalia, in so
far as physical development is concerned; and this, as we shall find,
accords with its remarkably genial climate and exuberant vegetation.
In Europe, the beds of this age present, for the first time, examples
of the monkeys, represented by two generic types, both of them
apparently related to the modern long-armed species, or Gibbons. Among
carnivorous animals we have cat-like creatures, one of which is the
terrible _Machairodus_, distinguished from all modern animals of its
group by the long sabre-shaped canines of its upper jaw, fitting it to
pull down and destroy those large pachyderms which could have easily
shaken off a lion or a tiger. Here also we have the elephants,
represented by several species now extinct; the mastodon, a great,
coarsely-built, hog-like elephant, some species of which had tusks
both in the upper and lower jaw; the rhinoceros, the hippopotamus, and
the horse, all of extinct species. We have also giraffes, stags, and
antelopes, the first ruminants known to us, and a great variety of
smaller and less noteworthy creatures. Here also, for the first time,
we find the curious and exceptional group of Edentates, represented by
a large ant-eater. Of all the animals of the European Miocene, the
most wonderful and unlike any modern beast, is the Dinotherium, found
in the Miocene of Epplesheim in Germany; and described by Kaup. Some
doubt rests on the form and affinities of the animal; but we may
reasonably take it, as restored by its describer, and currently
reproduced in popular books, to have been a quadruped of somewhat
elephantine form. Some years ago, however, a huge haunch bone,
supposed to belong to this creature, was discovered in the South of
France; and from this it was inferred that the Dinothere may have been
a marsupial or pouched animal, perhaps allied in form and habits to
the kangaroos. The skull is three feet four inches in length; and when
provided with its soft parts, including a snout or trunk in front, it
must have been at least five or six feet long. Such a head, if it
belonged to a quadruped of ordinary proportions, must represent an
animal as large in proportion to our elephant as an elephant to an ox.
But its size is not its most remarkable feature. It has two large
tusks firmly implanted in strong bony sockets; but they are attached
to the end of the lower jaw and point downward at right angles to it,
so that the lower jaw forms a sort of double-pointed pickaxe of great
size and strength. This might have been used as a weapon; or, if the
creature was aquatic, as a grappling iron to hold by the bank, or by
floating timber; but more probably it was a grubbing-hoe for digging
up roots or loosening the bases of trees which the animal might
afterward pull down to devour them. However this may be, the creature
laboured under the mechanical disadvantage of having to lift an
immense weight in the process of mastication, and of being unable to
bring its mouth to the ground, or to bite or grasp anything with the
front of its jaws. To make up for this, it had muscles of enormous
power on the sides of the head attached to great projecting processes;
and it had a thick but flexible proboscis, to place in its mouth the
food grubbed up by its tusks. Taken altogether, the Dinothere is
perhaps the most remarkable of mammals, fossil or recent; and if the
rest of its frame were as extraordinary as its skull, we have probably
as yet but a faint conception of its peculiarities. We may apply to
it, with added force, the admiring ejaculation of Job, when he
describes the strength of the hippopotamus, "He is the chief of the
ways of God. He who made him, gave him his sword."
[Illustration: MIOCENE MAMMALS OF THE EASTERN CONTINENT.
In the foreground _Elephas_, _Ganesa_, _Hydracotherium_,
_Dinotherium_, _Machairodus_, _Mastodon longirostris_. In the middle
distance, _Apes_, two _Anoplotheres_, _Palæotherium_, _Xiphodon_, and
_Sivatherium_. Sequoias and Fan Palm in the background.]
In Asia, the Siwalik hills afforded to Falconer and Cautley one of the
most remarkable exhibitions of Miocene animals in the world. These
hills form a ridge subordinate to the Himalayan chain; and rise to a
height of 2,000 to 3,000 feet. In the Miocene period, they were sandy
and pebbly shores and banks lying at the foot of the then infant
Himalayas, which, with the table-lands to the north, probably formed a
somewhat narrow east and west continental mass or large island. As a
mere example of the marvellous fauna which inhabited this Miocene
land, it has afforded remains of seven species of elephants,
mastodons, and allied animals; one of them, the _E. Ganesa_, with
tusks ten feet and a half long, and twenty-six inches in circumference
at the base. Besides these there are five species of rhinoceros, three
of horse and allied animals, four or more of hippopotamus, and species
of camel, giraffe, antelope, sheep, ox, and many other genera, as well
as numerous large and formidable beasts of prey. There is also an
ostrich; and, among other reptiles, a tortoise having a shell twelve
feet in length, and this huge roof must have covered an animal
eighteen feet long and seven feet high. Among the more remarkable of
the Siwalik animals is the _Sivatherium_, a gigantic four-horned
antelope or deer, supposed to have been of elephantine size, and of
great power and swiftness; and to have presented features connecting
the ruminants and pachyderms. Our restoration of this creature is to
some extent conjectural; and a remarkably artistic, and probably more
accurate, restoration of the animal has recently been published by
Dr. Murie, in the Geological Magazine. We justly regard the Mammalian
fauna of modern India as one of the noblest in the world; but it is
paltry in comparison with that of the much more limited Miocene India;
even if we suppose, contrary to all probability, that we know most of
the animals of the latter. But if we consider the likelihood that we
do not yet know a tenth of the Miocene animals, the contrast becomes
vastly greater.
Miocene America is scarcely behind the Old World in the development of
its land animals. From one locality in Nebraska, Leidy described in
1852 fifteen species of large quadrupeds; and the number has since
been considerably increased. Among these are species of Rhinoceros,
Palæotherium, and Machairodus; and one animal, the Titanotherium,
allied to the European Anoplothere, is said to have attained a length
of eighteen feet and a height of nine, its jaws alone being five feet
long.
In the illustration, I have grouped some of the characteristic
Mammalian forms of the Miocene, as we can restore them from their
scattered bones, more or less conjecturally; but could we have seen
them march before us in all their majesty, like the Edenic animals
before Adam, I feel persuaded that our impressions of this wonderful
age would have far exceeded anything that we can derive either from
words or illustrations. I insist on this the more that the Miocene
happens to be very slenderly represented in Britain; and scarcely at
all in north-eastern America; and hence has not impressed the
imagination of the English race so strongly as its importance
justifies.
The next succeeding period, that of the Pliocene, continues the
conditions of the last, but with signs of decadence. Many of the old
gigantic pachyderms have disappeared; and in their stead some familiar
modern genera were introduced. The Pliocene was terminated by the cold
or glacial period, in which a remarkable lowering of temperature
occurred over all the northern hemisphere, accompanied, at least in a
portion of the time, by a very general and great subsidence, which
laid all the lower parts of our continents under water. This
terminated much of the life of the Pliocene, and replaced it with
boreal and Arctic forms, some of them, like the great hairy Siberian
mammoth and the woolly rhinoceros, fit successors of the gigantic
Miocene fauna. How it happened that such creatures were continued
during the Post-pliocene cold, we cannot understand till we have the
Tertiary vegetation before us. It must suffice now to say, that as the
temperature was modified, and the land rose, and the Modern period was
inaugurated, these animals passed away, and those of the present time
remained.
Perhaps the most remarkable fact connected with this change, is that
stated by Pictet, that all the modern European mammals are direct
descendants of Post-pliocene species; but that in the Post-pliocene
they were associated with many other species; and these, often of
great dimensions, now extinct. In other words, the time from the
Pliocene to the Modern, has been a time of diminution of species,
while that from the Eocene to the Miocene was a time of rapid
introduction of new species. Thus the Tertiary fauna culminated in the
Miocene. Yet, strange though this may appear, Man himself, the latest
and noblest of all, would seem to have been a product of the later
stages of the time of decadence. I propose, however, to return to the
animals immediately preceding man and his contemporaries, after we
have noticed the Tertiary flora and the Glacial period.